The main reason the M84 scope was introduced was so that US snipers would have a scope that was more resistant to moisture than either the Weaver or Lyman predecessors. The M84 was designed by Ordnance and subsequently manufactured by Libby Owens Ford, Leupold and R. Rendell.
When ordnance prepared the written specifications for the M1903A4 it is clear they preferred the Lyman Alaskan Scope. Unfortunately for the sniping effort the Weaver was the scope that was available at the time so the M73B1 became the defacto standard.
However, as has been mentioned any of scopes mentioned are acceptable on an A4.